Lucy or Brucey? It Can Be Tricky to Tell the Sex of Fossil Ancestors

An artist's conception shows human ancestors from a long-extinct species called Australopithecus afarensis. The first specimen of the species, known as Lucy, was discovered 40 years ago.Michael Hagelberg / ASU Research Magazine

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Was Lucy a she? Paleoanthropologists have always visualized the famous skeleton of the human ancestor found 40 years ago as a petite female, due to the size of the bones. But now that a variety of specimens have been classified as Australopithecus afarensis, some researchers are saying it's harder than some people think to determine the sex of Lucy's kin.

"We can't really 'sex' Australopithecines," Penn State anthropologist Philip Reno said in a news release. The size variations in the long-extinct species' bones are such that scientists shouldn't always assume the smaller-sized skeletons belonged to females, Reno and Kent State evolutionary biologist C. Owen Lovejoy reported Tuesday in the open-access journal PeerJ.

On the left is a large, presumably male specimen of a femora bone from Australopithecus, and on the right is a corresponding bone from the Lucy skeleton. The difference between the two bones gives the impression of large size differences between the sexes. However, inclusion of the many intermediately sized specimens from this species indicates that the size variation is similar to moderately dimorphic humans.Phil Reno / Penn State

Sex-related differences in size are known as sexual dimorphism: Gorillas are highly dimorphic, while humans are only moderately so. The newly published analysis, based on a study of more than 40 specimens, determined that Australopithecines were more like humans than like gorillas when it came to size differences.

Despite the uncertainties, Lovejoy said Lucy was most likely female. "The reason we know she's female is that she's the smallest of the species that's ever been found," he told NBC News.

"The problem is, there's a whole host of specimens that are in the middle — and somewhere you get into a range where we just don't know," he added.

It's not always about size

Reno told NBC News that the typical way to determine the sex of human skeletons — by looking at the shape of their pelvises for childbearing — wouldn't work for Australopithecines, since they probably didn't have the birth-canal issues that humans have. (There's a debate about that, however.)

Some researchers had hoped that size would separate the males from the females, as it clearly does for gorillas. Reno and Lovejoy say that's not a reliable yardstick. "It's like saying that if you're taller than 5-foot-8, you must be a male," Lovejoy said. Reno, who says he's 5-foot-6, pointed out that "it's not uncommon to find women who are taller than me."

Other paleoanthropologists are not totally convinced.

"The contemporaneity of the First Family remains is strong vindication of sexual dimorphism," Donald Johanson, a paleoanthropologist at Arizona State University who unearthed Lucy's remains in 1974, told NBC News in an email. "If we have large afarensis and small afarensis fossils, and it is not due to sexual dimorphism, then what is the explanation?"

Johanson said the smallest specimens were most likely female, and the largest specimens were "undoubtedly male."

But he agreed with Reno and Lovejoy that the issue gets tricky somewhere in the middle: "There are certainly bones situated in the center of the normal curve that cannot be definitively placed in either sex."